1. Field of the Invention
The present invention relates to an equalizer for an optical reproducing apparatus which reproduces data by suitably performing a decoding operation on signals read from a disc-like recording medium in accordance with a partial-response transmitting method.
2. Description of the Related Art
In the field of a system using recording mediums, such as optical discs and magneto-optical discs, research is being made on increasing the recording capacity and the recording density. To achieve high density recording, it is important to shorten the laser wavelengths and to suitably set the numerical aperture (NA) of an objective lens. Moreover, the partial-response transmitting method in which the intersymbol interferences, which would disadvantageously act on high density recording, are paradoxically utilized, is also known as a digital-data transmitting method for facilitating high density recording.
Several types of partial-response transmitting methods may be classified by allocating data in accordance with the characteristics of the transfer functions. The above transmitting methods which have been put into practical use may be represented by PR0 (class 0 partial response, which is also referred to as "the PR(1) method") through PR5 (class 5 partial response, which is also referred to as "the PR (-1, 2, -1) method"), shown in FIG. 12. FIG. 12 illustrates the elements inherent in each partial response method, i.e., the response waveform, the frequency characteristics, and the output level number in response to the binary input data after PR equalization is performed.
In order to achieve even higher density recording after the laser wavelength and the NA are suitably set, it is necessary to introduce the foregoing partial-response transmitting method and also to perform an equalizing operation (correcting the frequency characteristics) on the transmitting signals by means of filtering in an electrical circuit. However, the above equalizing operation may impair the reliability of detecting the data. Thus, the electrical equalizing operation is not sufficient in order to achieve even higher density recording.
Further, in the above equalizing operation, the spatial frequency characteristics are corrected on a time axis. It is thus necessary to change the frequency characteristics in accordance with the linear velocity obtained during disc reproduction. For example, in a constant angular velocity (CAV)-type disc, or a zone CAV-type disc, the equalizing characteristics should be changed in accordance with the reproducing position in the radial direction of the disc. This inevitably increases the complexity of the configuration of the circuit and the equalizing operation.